1. Field of the Invention
Embodiments of the invention generally relate to a heart stimulator with a stimulation unit connected to a stimulation electrode to stimulate a ventricle of a heart.
2. Description of the Related Art
Typically, a stimulation unit is designed to generate both stimulation pulses and antitachycardic defibrillation shocks, and for this purpose has at least one high voltage capacitor in which the electricity required for an antitachycardic defibrillation shock may be stored. Generally, the heart stimulator also has a detector, which is designed to process physiological signals received from the heart, and on the basis of these signals detects the presence of an acute ventricular tachycardia or fibrillation. Typically, a further component of the heart stimulator is a control unit which is connected to the detector and the stimulation unit and which is designed to respond to an output signal of the detector, and to actuate the stimulation unit either to emit a train of stimulation pulses forming an antitachycardic therapy (antitachycardic pacing therapy, ATP) or an antitachycardic defibrillation shock.
Such heart stimulators are generally referred to as implantable cardioverter defibrillators (ICD's). The heart stimulators referred to herein are therefore typically primarily implantable heart therapy devices which are capable of treating a heart tachycardia.
Within the scope of this application tachycardias are generally understood to mean both tachycardias in the narrower sense, which are characterized by a stable cardiac rhythm with a pathologically high frequency, and fibrillations also called unstable tachycardia. Typical therapies, which may be provided by a heart stimulator of the type already mentioned, are antitachycardic stimulation or antitachycardic defibrillation shock.
An antitachycardic defibrillation shock is generally an electric current surge which is transmitted to the heart and which has a sufficiently high voltage and energy to fully excite a ventricle affected by fibrillation and therefore to render it refractory. Generally, circulating excitations typical of fibrillations are interrupted in this manner. In the case of a tachycardia in the narrower sense, which is often also referred to as ventricular tachycardia, if the ventricle is affected, and which is abbreviated to VT or stable VT (as opposed to ventricular fibrillation VF or unstable VT), successful therapy using antitachycardic stimulation (anti tachyarrhythmia pacing; ATP) is often possible, especially for stable VT. In antitachycardic stimulation, the heart stimulator typically emits a series of stimulation pulses whose energy is much lower than the energy of an antitachycardic defibrillation shock and which are generally not painful either. In antitachycardic stimulation, such stimulation pulses of comparatively lower energy are typically emitted with a frequency that exceeds the frequency of the recorded tachycardia. Generally, the anti tachyarrhythmia pacing is also called ATP or ATP one shot. Typically, in many cases a tachycardia may be terminated in this manner without the patient suffering pains or without the energy requirement being particularly high.
Since success with an antitachycardic stimulation is not always achieved, it may be necessary to give an antitachycardic defibrillation shock following unsuccessful antitachycardic stimulation (ATP).
For example, U.S. Pat. No. 6,718,204 entitled “Method and Apparatus to Control Delivery of High-Voltage and Anti-Tachy Pacing Therapy in an Implantable Medical Device”, to Degroot et al., discloses a method for giving such antitachycardic stimulation after a tachycardia is detected, during or shortly before a high voltage capacitor is charged, such that antitachycardic defibrillation shock may be given after the tachycardic stimulation if necessary.
In Degroot et al., there appears to be contradictory requirements of wanting to save as much as possible on the one hand, i.e. avoiding charging the high voltage capacitor wherever possible, and on the other hand being able to give an antitachycardic defibrillation shock as soon as possible after an unsuccessful antitachycardic stimulation, which in principle presupposes a capacitor that is already charged.
For example, a possible solution to the above mentioned dilemma may be found in United States Patent Publication 2007/0270911 entitled “Heart Stimulator”, to Doerr et al., which discloses discontinuing charging of a high voltage capacitor in case of a terminated tachycardia.
Generally, also relevant for the antitachycardia therapy are the criteria for discriminating a tachycardia from a fast but still tolerable heart rhythm or non-sustained arrhythmia to decrease the number of inadequate antitachycardia defibrillation shock therapies. For example, U.S. Pat. No. 8,229,563 entitled “Devices for Adapting Charge Initiation for an Implantable Cardioverter-Defibrillator”, U.S. Pat. No. 8,249,702 entitled “Method for Adapting Charge Initiation for an Implantable Cardioverter-Defibrillator”, United States Patent Publication 2012/0323290 entitled “Methods and Devices for Adapting Charge Initiation for an Implantable Defibrillator”, and United States Patent Publication 2012/0316612 entitled “Methods and Devices for Adapting Charge Initiation for an Implantable Defibrillator”, all to Warren et al., disclose an increasing X out of Y criteria, to enhance the reliability of arrhythmia detection.